The invention generally relates to mixed-signal converters of the sigma-delta noise shaping type, and more particularly, to mixed-signal digital-to-analog converters that employ uniformly weighted elements.
Sigma-delta digital-to-analog converters (DACs) provide for a means to achieve high resolution and low distortion at a relatively low cost compared to traditional Nyquist converters. In a typical multi-bit noise-shaped oversampling DAC, the digital input is first up-sampled by the oversampling ratio (OSR) and filtered to suppress the out-of-band images. A sigma-delta modulator is then used to reduce the word-width to a manageable size, and at the same time, shape the in-band noise to a higher frequency region. A binary to thermometer encoder is used to convert the binary data into thermometer-code data. For example, U.S. Pat. No. 5,404,142 discloses a data-directed scrambling technique in which a quantized noise-shaped word is first converted to a thermometer code. A data-directed shuffler is then used to dynamically select a group of elements of the output stage. The number of elements selected is equal to the number of active thermometer codes. An analog output stage then converts the output of the shuffler into an analog quantity by turning on the selected group of elements according to the decision of the shuffler.
A prior art thermometer-code DAC includes a current steering section and an I-to-V converter that includes a DAC cell driver that controls the BIT and {overscore (BIT)} (or BITB) signals. By designing the cross point of the BIT and BITB signals to be one Vgs above the common-mode voltage, the inter-symbol interference (ISI) in the DAC cell output waveform will be minimized. Vgs is defined to be the gate-to-source voltage of the DAC switches when each is conducting half of the output current.
Due to imperfection of real devices, the current cells will not match exactly. This mismatch problem results in harmonic distortion and noise in the reconstructed analog signal. The performance of the converter is thus limited by the matching of these elements. Commercially available silicon processes can only offer matching of up to 12 bits without calibration or trimming.
This element mismatch has been well studied and methods have been proposed to convert the mismatch error into spectrally shaped noise. By shaping the mismatch error into out-of-band frequency region, the signal-to-noise ratio (SNR) and dynamic range (DNR) of the converter is greatly improved. In these methods, a shuffler (also sometimes called scrambler) is used to dynamically select a group of elements for every digital input code such that over time, each element is equally used. This implies that the first integral of the difference between every pair of elements is zero, hence, equivalent to a first-order noise shaped sigma-delta converters. The only difference is in a normal sigma-delta converter, the amplitude error is noise shaped whereas in a data shuffler, the error in the usage of the element is noise shaped. An example of a prior art butterfly-style shuffler is disclosed in U.S. Pat. No. 6,614,377. A drawback, however, of conventional thermometer-code current steering DACs is thermal noise performance. In particular, when the data is zero, half the number of the switching current sources are connected to one summing junction, and the other half are connected to the other summing junction of the I-to-V converter. Moreover, the top current sources are always connected to the summing junctions. The current sources are the dominant noise source in the DAC output and dictate the SNR of the converter.
Another conventional DAC architecture that does not suffer from the mentioned noise problem includes tri-level logic thermometer current steering DAC that includes a pair of current sources (positive and negative) for each of the bits 0 to 15. Since each pair of current sources can be connected to the summing junction in three different ways, each pair may contribute a positive, a negative quantity of charge or nothing at all. When the data is zero, all the current sources are connected to a buffer amplifier to maintain their proper drain voltage. Therefore, the main noise source of the converter is now from the I-to-V amplifier, which by design, is much smaller than that of the current sources. Hence, the SNR is significantly improved. A difficulty with this architecture, however, is that it again may result in the element mismatch discussed above. Prior art element shufflers do not work with this architecture since they can only shuffle “1” and “0”.
There is a need, therefore, for an improved sigma-delta noise-shaping DAC that further reduces element mismatch.